Brown dwarf

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Artistic conception of a brown dwarf type-T

Comparison: most brown dwarfs are just bigger than Jupiter (10-15%), but they can be up to 75 times more massive due to their high density.

The brown dwarfs They are sub-stellar objects that are not massive enough to maintain continuous nuclear reactions of hydrogen-1 fusion at their core, unlike the stars of the main sequence. Brown dwarfs occupy the rank of masses between the heaviest giant gas planets and the lightest stars, with a relatively well-known upper mass limit, being understood between 75 and 80 Jordanian masses ${displaystyle M_{J}}$ according to the degree of metallicity. Brown dwarfs heavier than 13 ${displaystyle M_{J}}$ it is believed that they merge deuterium and that those over 65 ${displaystyle M_{J}}$ They merge lithium. From 65 ${displaystyle M_{J}}$ In addition to deuterium they also burn tritio. However, they hardly have chemical differentiation according to depth, as they have suffered at some point in their convection life from the surface to their center because of weak fusion reactions of residual isotopes.

The burning of deuterium occurs in its youth and is possible due to its low melting temperature, about 100,000 K. Since deuterium is a minority fuel that disappears quickly, such a reaction cannot sustain gravitational collapse. Brown dwarfs continue to shine for a while due to the residual heat from the reactions and the slow contraction of the matter that makes them up. Brown dwarfs will continue to contract and cool until they reach equilibrium. Brown dwarfs are thought to be "failed" stars, as they contain the same materials as a star like the Sun, but with too little mass to shine. They are very similar to gas planets; They're not quite planets, but they're not stars either.

The differences that define a very low mass brown dwarf of a gas giant (~13) ${displaystyle M_{J}}$ ) are questions of debate. One school of thought is based on formation; the other, on the physics of the interior. A part of the concerns of the debate is whether the "baby browns" should, by definition, have experienced the merger at some point in their history.

Stars are classified by spectral class, with brown dwarfs designated as M, L, T, and types. and. Despite their name, brown dwarfs are different colors. Many brown dwarfs are likely to appear magenta to the human eye, or possibly orange or red. Brown dwarfs are not very luminous at visible wavelengths.

Some known planets that orbit brown dwarfs are: 2M1207 b, MOA-2007-BLG-192-L b, and 2MASS J044144.

At a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary brown dwarf system discovered in 2013. DENIS-P J082303.1-491201 b is listed as the most massive exoplanet known (as of March 2014) in the NASA exoplanet archive, despite having a mass (28.5 ± 1.9 ${displaystyle M_{J}}$ ) more than twice that the cutting point of 13 Jordanian masses between planets and brown dwarfs.

Historical Notes

Image of Gliese 229B, a brown dwarf.

In 1963, the Indian-born astrophysicist Shiv Kumar was the first to theoretically study the evolution and properties of stars with a much lower mass than those known at that time. His calculations would correspond to what we now call brown dwarfs. Kumar called these objects black dwarfs. The name Brown Dwarf was proposed in 1975 by astrophysicist Jill Tarter, famous for being the spokesperson for the SETI project. There is a discrepancy about the use of the names enana marrón or enana café worldwide, although in Spanish enana marrón has been imposed mainly. The first verified brown dwarf was Teide 1, in 1995, at the IAC-80 telescope of the Teide Observatory (Tenerife), by a Spanish group of astrophysicists belonging to the IAC and directed by Rafael Rebolo López. The best characterized brown dwarf is Gliese 229B, the lowest-mass companion to the star Gliese 229.

Lithium identification and testing

This illustration shows the size of our Sun compared to a brown dwarf, Jupiter and Earth.

Lithium is an especially interesting element to differentiate these objects from low-mass stars, since it is rapidly destroyed in hydrogen fusion reactions (particularly in PPII chains) because it reacts with hydrogen at such only 1,000,000 K. Low-massive stars (< 0.1M_S) are entirely convective, so their interiors are well mixed and lithium burns along with hydrogen in a few million years. In contrast, stars with masses similar to or greater than that of the Sun do maintain lithium in their atmospheres, since convection fails to penetrate to the core. In a brown dwarf, due to its low mass, hydrogen does not reach the temperatures and pressures necessary to trigger its fusion and lithium is not destroyed, remaining in the brown dwarf throughout its existence. This lithium can be detected through its characteristic emission spectra, constituting this test the classic way of identifying brown dwarfs. The test was used for the first time by the IAC group led by Rafael Rebolo López. The proof is not perfect, since there may be very low-mass stars in which the burning of lithium has not yet finished due to their slow reactions. The possibility of measuring the absence of deuterium to verify whether it is a brown dwarf or not was also considered, but this data turned out to be more difficult to measure, since the spectral lines of hydrogen and deuterium are very similar.

Brightness and spectral type

The low energy emission from these stars makes it very difficult to observe them directly from great distances. Despite this, several hundred brown dwarfs have been identified, with surface temperatures ranging between 800 and 2000 degrees Celsius. The surface temperature is an increasing function with the mass and decreasing with the age of the object.

Brown dwarfs and extrasolar planets

The artist's impression of a disc of dust and gas around a brown dwarf.

Because of their low mass, intermediate between giant planets and low-mass stars, brown dwarfs are a unique link between the two types of bodies. In particular, the formation of brown dwarfs is unknown, and it is not possible to know for the moment if they form as planets inside a circumstellar disk from a core of solid material, or as stars from fragmentation and gravitational collapse. of a molecular cloud. In 2003, a group of brown dwarf-type objects with masses as small as 5 Jovian masses was detected in the constellation Orion.

Planetary-mass super-Jupiter objects 2M1207b and 2MASS J044144 that orbit brown dwarfs at large orbital distances may have formed by cloud collapse rather than accretion and thus may be underwater brown dwarfs rather than planets, which follows from relatively large masses and large orbits. The first discovery of a low-mass companion orbiting a brown dwarf (ChaHα8) at a small orbital distance using the radial velocity technique paved the way for the detection of planets around brown dwarfs in orbits of a few AU or smaller. However, with a companion to primary mass ratio in ChaHα8 of about 0.3, this system looks more like a binary star. Then, in 2013, the first planetary-mass companion (OGLE-2012-BLG-0358L b) in a relatively small orbit was discovered orbiting a brown dwarf. In 2015, the first Earth-mass planet was found orbiting a brown dwarf, OGLE-2013-BLG-0723LBb.

The disks around brown dwarfs have been found to have many of the same characteristics as the disks around stars; Therefore, it is expected that there will be accretionary planets around brown dwarfs. Given the small mass of brown dwarf disks, most of the planets will be terrestrial planets rather than gas giants. If a giant planet orbits a brown dwarf through our line of sight, because they are about the same diameter, this would give a good signal for detection by the transit. The accretion zone for planets around a brown dwarf is very close to the brown dwarf itself, so tidal forces would have a strong effect.

Planets around brown dwarfs are probably waterless carbon planets.

A 2016 study, based on observations with Spitzer estimates that 175 brown dwarfs need to be monitored to guarantee (95%) at least one detection of a planet.

Livability

The possible habitability of planets orbiting a brown dwarf has been studied, and these studies show that the conditions for one of these stars to have a habitable planet are extremely strict due to both the fact that the habitable zone is very narrow (and also narrowing and getting closer to the brown dwarf as the star cools over time), as well as the fact that in particular the orbital eccentricity of such planets must be extremely low (of the order of 10^-6) to avoid that tidal forces are created that end up producing an uncontrolled greenhouse effect in them that makes them uninhabitable.

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